1. Field of the Invention
The present invention relates to testing equipment for internal combustion engines. In particular, the present invention relates to a tachometer for determining the engine speed (RPM) of an internal combustion engine of a vehicle.
2. Description of the Prior Art
In newer model automobiles, tachometer (RPM) measurement methods have become a problem because it is impossible to connect engine speed transducers to some vehicles, and extremely difficult to connect transducers to others.
The distributorless ignition system (DIS) used in newer model vehicles exhibits these problems. DIS systems have dual firing voltage on each spark plug wire, one for normal firing and the other for what is known as "waste firing". The prior art secondary type transducers pick up both spark plug firings, and the measuring equipment must distinguish these signals in order to develop a proper tachometer reading. The secondary signals sensed by these prior art transducers are not considered to be as reliable as one would like, since spark plug condition and cross firing can create undesirable or unreliable information.
In the prior art, it was customary to use the primary voltage or current as a source of tachometer information. The DIS systems, however, are typically packaged so that they do not have available a connection for sensing primary voltage. For that reason, a special current pickup was developed which would use the primary current when available. There are, however, vehicles in which even current readings have become difficult because the wire is bunched and harnessed in a complete cable.
There is a need for an improved tachometer which is compatible with the modern DIS systems, and which does not require special transducers, and which is compatible with both distributor and distributorless ignition systems.